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Old 20th Jul 2021, 2:24 pm   #1
Andy GD0TEP
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Default AVO 8 Mk5, should I attempt repair, or send it to a repairer?

Hi to the group,

I'm in the middle of fault finding an RF amplifier, and in the process, i managed to knock my trusty AVO 8 off the bench. It fell about 3 foot onto a solid floor. i turned to see it fall, all in slow motion (My digital fluke would have bounced...)

Now it no longer works.

Both batteries are ok and in place, and the fuse is ok. There's nothing obvious and it doesn't rattle.

I'm now stuck as i can't progress the amplifier as i will need to check 2,600v DC I also have another AVO 8, but it only goes up to 1,000v so...

Do i try and fix something that I've never opened before, or send to some one (who though??)

The meter wont measure anything, V, A, or R so i'm guessing it's something common to everything... the meter needle still appears to be free moving..

Any help or advise would be appreciated.

Thanks.

Andy
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Old 20th Jul 2021, 2:46 pm   #2
lesmw0sec
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Default Re: AVO 8 Mk5, should i attempt repair, or send it?

I would suggest a quick look. It might me nothing more than a snapped wire. I am suprised that it broke. I recall mixing with a bunch of horses when motocycling in the dark & rain in the New Forest. My Avo went flying over my head, landing about 100 yds. further on.
Later checks revealed no fault whatever!
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Old 20th Jul 2021, 2:54 pm   #3
Boulevardier
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Default Re: AVO 8 Mk5, should i attempt repair, or send it?

Sorry to point out the obvious, but you have checked that the cut-out button hasn't triggered to "off" with the mechanical shock..? I've been caught out by that many times!

Mike
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Old 20th Jul 2021, 4:02 pm   #4
pmmunro
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Default Re: AVO 8 Mk5, should i attempt repair, or send it?

It's a good sign that the pointer still swings freely, as long as it is not too free, indicating a lack of damping. If the pointer swings without the usual damping, that would indicate that the movement is either open-circuit or disconnected from the universal shunt chain.

It would certainly help if you had the service manual for the Model 8 Mk V. If you can't find that on-line, let me know through a private message and I will send it to you. Once you understand the curcuit of the meter it will be much easier to find the faul(s).

Basically these meters are 37.5 micoampere instruments with the rest of the circuits to give the various ranges calculated around that basic movement. If you can verify that the movement is operational, it should not be too difficult to find the trouble.

These meters use flexible printed wiring which has to be folded back to access the movement. This printed wiring is quite fragile and the tracks are easily broken if not handled carefully. Broken tracks can be by-passed and it is possible to make up a loom using conventional wiring to replace the print.

PMM
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Old 20th Jul 2021, 5:18 pm   #5
technotel
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Default Re: AVO 8 Mk5, should I attempt repair, or send it to a repairer?

Heres a couple of pdf's I have of mk5. Hope they help?
Terry
Attached Files
File Type: pdf Avometer8MkV.pdf (1.68 MB, 44 views)
File Type: pdf avo_8mkv_analog_mm_sm (2).pdf (801.9 KB, 41 views)
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Old 20th Jul 2021, 5:19 pm   #6
technotel
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Default Re: AVO 8 Mk5, should I attempt repair, or send it to a repairer?

I managed to repair mine. It had o/c track on x100 pot.
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Old 20th Jul 2021, 6:51 pm   #7
Sinewave
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Default Re: AVO 8 Mk5, should I attempt repair, or send it to a repairer?

How free does it move when the range switches are set OFF and DC, compared with other switch settings?
__________________
Avometer, vintage Fluke and Marconi collector.

At 144-146.00Mhz, keeping one wavelength apart.
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Old 20th Jul 2021, 7:55 pm   #8
Andy GD0TEP
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Default Re: AVO 8 Mk5, should I attempt repair, or send it to a repairer?

OK, quite a few suggestions, and ideas. Thanks guys for that.. I really appreciate it

Firstly, with the meter ‘off’ and I twist the meter left & right, the needle appears to move freely up about 10mm and then back again. If I turn one of the dials so it’s on volts, or similar, the needle would seem to be more damped with it returning to zero a lot better.

I hadn’t thought about the cut out button, and I really thought it was going to be one of those ‘how could I have missed it’ moments, but alas, it was not to be.. the cut-out is ok.

Thanks for the PDF files, very handy and very much appreciated.!

Hopefully I’ll be able to get the back off later tonight, or tomorrow… this depends on my duties as a grandparent to a one year old…

Will report back once I've got the back off and if I can see anything...
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Old 20th Jul 2021, 8:51 pm   #9
Electronpusher0
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Default Re: AVO 8 Mk5, should I attempt repair, or send it to a repairer?

I have a scrap Avo 8 Mk5 (movement magnet broke free), if you need any parts let me know by PM.

Peter
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Old 21st Jul 2021, 7:15 pm   #10
Andy GD0TEP
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Default Re: AVO 8 Mk5, should I attempt repair, or send it to a repairer?

Hello all…

So, some news…

When I looked at the circuit, there wasn’t much that it could have been. Boulevardier Hit the nail on the head, sort of…

When I got the back of the meter off I heard a rattle but couldn’t say where it was from… I removed the Movement Assembly, and then lifted the Component Printed circuit board. The rattle came from under the clear plastic cover that is over the meter movement. It’s now I noticed that the meter has been apart before as there was burn marks from a soldering iron on the clear plastic. It was also cracked and splintered from one of the cover screw holes up to the top of the cover (I guess only to be expected from an eBay sourced AVO of this age)

I guess that this is what remains of the cut-out?

What to do next though??
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Old 21st Jul 2021, 10:21 pm   #11
pmmunro
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Default Re: AVO 8 Mk5, should I attempt repair, or send it to a repairer?

These are parts of the cut-out. As Electronpusher has offered you a meter for parts, this would probably be your best option.

The clear plastic covers over the movement are very often cracked;this is due to deterioration of the plastic rather than mechanical damage.

PMM
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Old 22nd Jul 2021, 2:56 am   #12
prumpfy
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Default Re: AVO 8 Mk5, should I attempt repair, or send it to a repairer?

Hi guys,
Am new here.
I have an AVO model 8 MK2.
Was new in 1959. A long story;BUT
The MK2 had a 37.5 uA meter movement, and c cursory glance at the manual from posted by technotel suggests that not much has changed in 60 years.
The current shunt system provides the damping for the meter movement and converts the instrument to a 20,000 ohm per volt meter.
If the meter connection is disconnected, the meter movement will swing freely as suggested in post #4. if the movement is then shunted with a 6k8 resistor, the meter swing should reduce to the required/normal amount of swing. If the amount of swing doesnt change between with the resistor connected, then the meter coil is open circuit.
If this all works out OK then you have eliminated all of your worries. Just persevere with fault tracing.
My model 8 MK2 failed with the fine wire connection at the 6K8 shunt resistor. A 1.3 volt battery measured 1.6 volt. resoldering the broken wire fixed the problem.
Good luck and just take things slowly.
To measure 2500 volt, just add 30 megohms in series with the 1Kv terminal.

As a comment, a mate who worked in the telephony equipment division at Philips Hendon works in Adelaide, said the AVO was the only meter he ever trusted.
I got mine when it was fully depreciated at Philips. The instrument tech guys fitted new jewels and calibtrated it and it only comes out of its box on special occasions.
Hope this helps,
prumpfy
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Old 22nd Jul 2021, 7:06 am   #13
Andy GD0TEP
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Default Re: AVO 8 Mk5, should I attempt repair, or send it to a repairer?

Yeah, thanks for that… I sent a PM to him last night… No reply yet, but I’m sure it will come..

What I’m now wondering is that the cut-out (and REVMC)and perhaps the pointer movement? All look to be mounted on a metal plate… do I have to remove that or can that be left as is, and the cut-out removed ‘as is’ I shall have a better look later…
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Old 22nd Jul 2021, 8:31 am   #14
Electronpusher0
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Default Re: AVO 8 Mk5, should I attempt repair, or send it to a repairer?

Hi, I have just dug out the AVO, it turns out to be Avo 8 Mk6 not a 5.
Not sure if the cutout is the same but you are welcome to have it for the cost of posting.
One of the terminal posts has been removed for another forum member and as I said the meter movement is beyond repair as the central magnet has come loose.

Peter
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Old 22nd Jul 2021, 5:09 pm   #15
Andy GD0TEP
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Default Re: AVO 8 Mk5, should I attempt repair, or send it to a repairer?

Quote:
Originally Posted by Electronpusher0 View Post
you are welcome to have it for the cost of posting.

One of the terminal posts has been removed for another forum member and as I said the meter movement is beyond repair as the central magnet has come loose.

Peter
OK Peter,

Sounds OK and I'm sure it will fit, if you can find out what the postage is and I'll get funds to you. if you can reply to my PM with payment info.

Thanks very much!

Andy
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Old 22nd Jul 2021, 6:50 pm   #16
Electronpusher0
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Default Re: AVO 8 Mk5, should I attempt repair, or send it to a repairer?

OK Andy I will PM you tomorrow with the cost.

Peter
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